Nickel silicides formed by low-temperature annealing of compositionally modulated multilayers

ABSTRACT

Methods are disclosed for making a compound of nickel and silicon. According to an embodiment, on a surface of a substrate (e.g., silicon), multiple layer pairs are formed in a superposed manner. Each layer pair includes a respective layer of nickel and a respective layer of silicon each being 3 nm or less in thickness. The layers of nickel and silicon in the multiple layer pairs are formed in alternating order, thereby forming a multilayer structure, wherein the layers of nickel and silicon in the multilayer structure are formed at respective thicknesses corresponding to desired mole fractions of nickel and silicon in the multilayer structure. The multilayer structure is annealed at a temperature of 200° C. or less to form an amorphous alloy of nickel and silicon in the multilayer structure, wherein the alloy has the desired mole fractions of nickel and silicon. The amorphous alloy is allowed to nucleate and form a corresponding crystalline alloy having the desired mole fractions of nickel and silicon.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of the earlier priority date of U.S.provisional patent application No. 60/414,010, filed on Sep. 26, 2002,which is incorporated herein by reference.

FIELD

This disclosure concerns solid-state structures of silicon and nickeland methods for making and using these structures.

BACKGROUND

A great need exists for new materials for use in the fabrication ofmicroelectronic devices such as integrated circuits, displays, and thelike. Many of the materials currently being used for these applicationshave limitations that can diminish their effectiveness in devices ofwhich the dimensions of active circuit elements are at or near thecurrently achievable critical dimensions. Hence, there is a persistentneed for new materials that can be used under increasingly demandingprocess conditions. For example, there is a need for new materials thatcan be used for forming electrical interconnections under conditions inwhich modern microelectronic devices will have to be manufactured. Inother words, new materials are required that exhibit satisfactorymechanical and electrical properties when formed or otherwise used underconditions of ever-increasing miniaturization of active circuit elementsand their interconnections.

A key impetus behind the development of new materials is the need todecrease contact resistivity between active elements and adjacent metallayers. For example, the sources, gates, and drains of MOS transistorswithin a microelectronic device must be connected to metal layers thatdefine conductive interconnections allowing the transistors tocommunicate with each other and with external devices. Aluminum andcopper currently are the most common materials used for forming thesemetal interconnecting layers. Silicon, on the other hand, is the mostcommon material used for forming active circuit elements (e.g., sources,gates, and drains of MOS transistors). If aluminum is deposited directlyonto a source, gate, or drain of a MOS transistor, the aluminum candiffuse into the silicon and destroy the transistor. To avoid thisproblem, a separate metal layer usually is formed between the silicon ofthe active element and the aluminum of the metal-interconnect layer as adiffusion barrier. Unfortunately, the currently most effective barriermetals (tungsten and molybdenum) have high respective contactresistances to silicon. High contact resistance between the barrierlayer and the underlying silicon adversely affects the performance ofthe microelectronic device and leads to destructive heat generationduring operation of the device.

Today, silicides (alloys of silicon and at least one metal) often areformed below the barrier layer to improve conductivity between thebarrier layer and the underlying silicon. Silicides are advantageous forthis purpose because they exhibit low contact resistance to both siliconand the commonly used barrier materials. The silicide typically isformed by depositing a thin metal film on an area of exposed (bare)silicon associated with a respective active circuit element. During orafter deposition of the metal, a reaction between the metal and thesilicon occurs by establishing annealing conditions. Titanium and cobaltare currently the most commonly used metals for forming silicides insilicon-based microelectronic devices. Unfortunately, both theseelements have serious limitations. Hence, new silicide materials areneeded for current and next-generation applications inmicroelectronic-device fabrication.

One of the desired properties for a silicide is low resistivity. Forexample, the C54 phase of TiSi₂ has an attractively low resistivity of13-16 μΩ·cm, compared to CoSi₂ and NiSi each having a resistivity in therange of 14-20 μΩ·cm. Even though TiSi₂ exhibits lower resistivity thanCoSi₂ or NiSi, TiSi₂ is difficult to form at the smaller dimensionsutilized in modern microelectronic devices. For example, at line widthsof less than 0.20 μm, nucleation of the C54 phase of TiSi₂ does notoccur in conventional methods. Instead, TiSi₂ crystallizes as a C49phase, which has a relatively high resistivity of 60-70 μΩ·cm. Anotherproblem with forming TiSi₂ is agglomeration. At high annealingtemperatures, TiSi₂ forms separate agglomerates that substantiallyincrease the resistivity between the barrier metal and the underlyingsilicon. CoSi₂ does not exhibit agglomeration or these nucleationproblems at narrow line widths, but this compound also has undesirableproperties. Particularly, CoSi₂ can diffuse into the underlying activearea, causing a rough silicon-silicide interface and current leakageacross the junction. Formation of CoSi₂ also consumes a relatively largeamount of the underlying silicon, which can cause problems in deviceshaving shallow junction depths.

Nickel suicides have been considered as potential next-generationcontact materials for silicon microelectronic devices, but specificnickel-silicide compounds are excessively difficult or impossible toform using conventional methods. As with other suicides, multiple phasesof nickel silicide tend to form by a nickel-silicon solid-statereaction. In a conventional method for forming nickel silicide, thefirst nucleated phase formed in the metal-silicon solid-state reactionusually is not the most desirable silicide. For example, in aconventional method, a high-resistivity dinickel silicide phase (Ni₂Si)usually forms first, whereas nickel monosilicide (NiSi) is usually thedesired phase.

In one conventional method for forming phase-pure silicide contacts formicroelectronic applications, alternating annealing and etching stepsare performed. For example, a nickel layer is deposited on silicon,followed by annealing to form crystalline Ni₂Si at the nickel-siliconinterface. Excess nickel is etched away and the remaining structure isre-annealed to change the silicide phase to NiSi. The annealingtemperatures required for these steps are often the highest temperaturesused in the microelectronic-device-fabrication process. With increasingminiaturization of microelectronic devices, these higher temperatures nolonger can be tolerated, necessitating the need for lower-temperaturesteps capable of forming the desired silicide compound.

A second conventional method for forming phase-pure silicide contactsfor microelectronic applications involves the deposition of a thin,interfacial layer between the silicon and the overlying metal layer,followed by annealing. The interfacial layer helps modulate the firstnucleated phase by limiting the amount of metal available for reactionat the interface. Exemplary conventional interfacial layers that havebeen investigated comprise titanium or a combination of palladium andplatinum. While such an interfacial layer can facilitate the formationof NiSi at a contact of Ni with Si, the interfacial layer also tends toform one or more other, unwanted, phases that compromise the resistivityof the contact. This problem becomes more pronounced with decreasingdimensions of the contact.

SUMMARY

The foregoing and other shortcomings of conventional methods aresatisfied by compositions and methods as disclosed herein. According toa first aspect, compositions of matter are disclosed. An embodiment ofsuch a composition comprises a substrate and a multilayer structureformed on the surface of the substrate. The multilayer structurecomprises multiple superposed layer pairs, wherein each layer pairconsists of a first layer of silicon and a second layer of nickel andhaving a layer-pair thickness of 3.0 nm or less.

The substrate can be any of various materials such as a semiconductormaterial, a metal, a glass material, a crystalline material, and aceramic material. For example, the surface of the substrate can be asurface of a silicon layer applied to the substrate. Alternatively, thesubstrate can be silicon in any of various forms, including but notlimited to doped silicon.

The composition desirably exhibits an electrical conductivity, from themultilayer structure to the substrate, of less than 13 μΩ·cm.

The multilayer structure desirably comprises two to ten layer pairs anddesirably comprises equal molar percentages of nickel and silicon.Alternatively, the multilayer structure can comprise more than 50mole-percent of nickel. The multilayer structure can be amorphous NiSi,with an optional capping layer (desirably of nickel), or crystallineNiSi, with an optional capping layer (again, desirably of nickel).

According to another aspect, methods are provided for making a compoundof nickel and silicon. In an embodiment of the method multiple layerpairs are formed on the surface of a substrate in a superposed manner.Each layer pair comprises a respective layer of nickel and a respectivelayer of silicon each being 3 nm or less in thickness. The layers ofnickel and silicon in the multiple layer pairs are formed in alternatingorder, thereby forming a multilayer structure, wherein the layers ofnickel and silicon in the multilayer structure are formed at respectivethicknesses corresponding to desired mole fractions of nickel andsilicon in the multilayer structure. In another step the multilayerstructure is annealed at an annealing temperature of 200° C. or less toform an amorphous alloy of nickel and silicon in the multilayerstructure, wherein the alloy has the desired mole fractions of nickeland silicon. In yet another step the amorphous alloy is allowed tonucleate and form a corresponding crystalline alloy having the desiredmole fractions of nickel and silicon. The substrate can be any ofvarious materials as summarized above. If necessary or desired, thesurface of the substrate (e.g., a silicon surface) is cleaned beforeforming the multilayer structure on the silicon surface.

The step of allowing the amorphous alloy to nucleate desirably isperformed by annealing the amorphous alloy at an annealing temperatureof 350° C. or less. Desirably, at onset of annealing, the annealingtemperature is ramped up to the annealing temperature.

As noted above, the number of layer pairs is two to ten, but the numbercan be as great as necessary or desired. The layers of silicon andnickel desirably are formed at respective thicknesses sufficient to formthe multilayer structure having substantial equal mole percentages ofnickel and silicon. Alternatively, the layers of silicon and nickel areformed at respective thicknesses sufficient to form the multilayerstructure having more than 50 mole-percent of nickel. Each of the nickellayers and each of the silicon layers can be formed by a suitabletechnique such as electron beam evaporation.

The method further can comprise the step of forming a capping layersuperposedly on the multilayer structure. The capping layer can be, forexample, a layer of nickel, typically formed at a thickness greater thana nickel-layer thickness in the multilayer structure.

According to another aspect, methods are provided (in the context of amicroelectronic-device fabrication method) for providing alow-resistivity contact to a silicon-containing active-circuit element(e.g., to a metal conductor). In an embodiment of such a method,multiple layer pairs are formed in a superposed manner on a region ofthe surface of the active-circuit element. Each layer pair comprises arespective layer of nickel and a respective layer of silicon each being3 nm or less in thickness. The layers of nickel and silicon in themultiple layer pairs are formed in alternating order, thereby forming amultilayer structure, wherein the layers of nickel and silicon in themultilayer structure are formed at respective thicknesses correspondingto desired mole fractions of nickel and silicon in the multilayerstructure. In another step the multilayer structure is annealed at anannealing temperature of 200° C. or less to form an amorphous alloy ofnickel and silicon in the multilayer structure, wherein the alloy hasthe desired mole fractions of nickel and silicon. In yet another stepthe amorphous alloy is allowed to nucleate and form a correspondingcrystalline alloy having the desired mole fractions of nickel andsilicon.

The method can comprise the step, after forming the crystalline alloy,of connecting a metal conductor to the crystalline alloy. This step canbe performed on a capping layer applied to the multilayer structure. Thestep of allowing the amorphous alloy to nucleate desirably is performedby annealing the amorphous alloy at an annealing temperature of 350° C.or less. As noted above, the annealing temperature desirably is reachedby ramping.

The method further can comprise the step of cleaning the surface of theactive-circuit element before forming the multilayer structure on thesilicon surface.

The foregoing and additional features and advantages will be morereadily apparent from the following detailed description, which proceedswith reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view of a first representative embodiment of anickel-silicon multilayer structure.

FIG. 2 is an elevational view of a second representative embodiment of anickel-silicon multilayer structure.

FIG. 3 is an elevational view of an exemplary intermediate structureformed during annealing of the structure of either FIG. 1 or FIG. 2.

FIG. 4 is an elevational view of an exemplary product formed uponcompleting annealing of the structure of either FIG. 1 or FIG. 2.

FIG. 5 is a plot of exemplary grazing-angle diffraction data obtained inthe Example from as-deposited samples.

FIG. 6 is a plot of exemplary grazing-incidence diffraction data as afunction of annealing temperature for sample J052 in the Example.

FIG. 7 is a plot of exemplary grazing-incidence diffraction data as afunction of annealing temperature for sample J053 in the Example.

FIG. 8 is a plot of exemplary grazing-incidence diffraction data as afunction of annealing temperature for sample J054 in the Example.

DETAILED DESCRIPTION

The present disclosure encompasses, inter alia, methods for preparingbinary and higher phases of nickel (Ni) and silicon (Si) usingcompositionally modulated multilayers as reactive precursors. Thedisclosure also encompasses the Ni—Si multilayer structures. The term“multilayer” herein denotes the result of a method in which layers of Siand Ni are alternatingly deposited on a substrate in a sequentialmanner, yielding multiple superposed layer pairs. It will be understoodthat the actual structure of the resulting composition does notnecessarily exhibit individually defined layers. Hence the term“multilayer” reflects the sequential deposition procedure used forforming the structure rather than the actual structure of the resultingcomposition. In some cases the actual structure of the resultingcomposition will exhibit substantial stratification; in other casesstratification will be less pronounced; and in yet other cases thestructure formed on the substrate will be amorphous and uniform.

A first representative embodiment of a multilayer structure 10 is shownin FIG. 1. The depicted multilayer structure 10 comprises a siliconsubstrate 12, an overlying multilayer structure 14, and a Ni cappinglayer 16 overlying the multilayer structure 14. The multilayer structure14 comprises multiple layer pairs 18. Each layer pair 18 comprises arespective layer of nickel 20 and a respective layer of silicon 22. Eachnickel layer 20 and each silicon layer 22 desirably is of the respectiveelement in substantially pure form. In this embodiment, the nickellayers 20 and the silicon layers 22 contain approximately equimolaramounts of the respective element, but are not equal in thickness. Thedifference in the thickness of nickel layers 20 and silicon layers 22reflects the difference in molar volume between the two elements. Thecapping layer 16 desirably is thicker than any one of the nickel layers20.

A second representative embodiment of a multilayer structure 30 is shownin FIG. 2, in which features that are similar to corresponding featuresin FIG. 1 have the same respective reference numerals. The multilayerstructure 30 is similar to the multilayer structure 10 in FIG. 1, butincludes thicker nickel layers 20 and no capping layer 16. The totalnickel content of the multilayer structure 14, in this embodiment, issimilar to the total nickel content of the combination of the multilayerstructure 14 and the nickel capping layer 16 in the embodiment of FIG.1.

A third representative embodiment of a multilayer structure 40 is shownin FIG. 3, which depicts a possible intermediate structure formed duringannealing of the multilayer structure 10 in FIG. 1 or the multilayerstructure 30 in FIG. 2. The depicted multilayer 40 comprises a siliconsubstrate 12 and an overlying nickel-rich amorphous region 24.

A fourth representative embodiment of a multilayer structure 50 is shownin FIG. 4, which depicts a final product created by annealing themultilayer structure 10 in FIG. 1 or the multilayer structure 30 in FIG.2. The post-anneal structure 50 comprises a silicon substrate 12 and anoverlying layer 26 of NiSi.

A representative embodiment of a method for making a compositionallymodulated multilayer structure is performed on a suitable substrate suchas silicon.

Alternatively, the substrate can be any other suitable solid materialsuch as another semiconductor material, a glassy material, a crystallinematerial, or a ceramic material. The substrate may or may not have anoverlying layer of dielectric, doped semiconductor, or the like,according to prevailing requirements. In manymicroelectronic-device-fabrication methods that include the subjectmethod for making a compositionally modulated multilayer structure, thesilicide will be formed on exposed silicon, e.g., over the junctions andgates of transistors, following the formation of the sources, drains,gates, and oxide spacers (as required) of the transistors. However, itwill be understood that the multilayer structures can be formed on anyof various surfaces of silicon or other suitable substrate. Thedescription below is in the context, by way of example, of using siliconas the substrate.

The method typically begins by preparing the substrate 12, which in thisinstance is silicon or another material on which a silicon layer hasbeen formed. This step of preparing the substrate can include, forexample, stripping native oxide from the silicon surface using astandard wet-clean process. Preparation of the surface of the substrate12 also or alternatively can include subjecting the surface to acleaning process suitable for removing particles from the surface.

In a next step of the method, the compositionally modulated multilayerstructure 14 is formed. This step can be accomplished by any of variousmultisource-evaporation processes, the most desirable beingelectron-beam evaporation, in which the substrate 12 is first placedinto a multisource-deposition chamber. The chamber includes respectiveelectron guns that transfer material from first and second elementalsources (i.e., Ni and Si sources) to the substrate 12. Deposition ratesof the elements onto the substrate 12 can be monitored in situ using aquartz-crystal microbalance or monitored ex situ by x-ray reflectivity.The desired modulation of respective concentrations of Ni and Si in themultilayer structure 14 can be achieved by alternating the deposition ofthe Ni and Si. After formation of the compositionally modulatedmultilayer structure 14, a capping layer 16 (e.g., a thick layer of Ni)can be deposited in the same manner, using a Ni source in the chamber.

The compositionally modulated multilayer structure 14 is subjected to afirst annealing step to convert the region into phase-pure NiSi. Any ofvarious annealing conditions will achieve the desired conversion. Forexample, annealing at 350° C. for one hour with a staged ramp-up intemperature is effective for converting the multilayers in the region 14into phase-pure NiSi. This first annealing step can be carried out in astandard furnace.

It is theorized that, during this first annealing step but prior tocrystallization of NiSi in the region 14, the individual Ni layers 20and Si layer 22 interdiffuse with each other and with the capping layer16 to form an amorphous phase 24. Ni diffuses rapidly through theamorphous phase 24, whereas the diffusion rate of amorphous NiSi intosilicon is slower. Thus, at a moderate annealing temperature, Ni isexpected to diffuse readily in the capping layer 16 and in the layers20, 22 of the multilayer structure 14 without appreciable diffusion ofNi at the interface between the multilayer structure 14 and thesubstrate 12. After concluding the first annealing step, a secondannealing step is performed at a higher annealing temperature than thefirst annealing step. The second annealing step causes nucleation ofpure NiSi (as a first nucleated phase) in the amorphous phase 24,beginning at the interface between the amorphous phase 24 and thesilicon substrate 12.

In an alternative scenario, the individual layers 20, 22 in themultilayer structure 14 interdiffuse with each other, but not with thecapping layer 16, to form an amorphous phase 24 containing approximatelyequivalent concentrations of nickel and silicon. In this scenario,nucleation that begins within the amorphous phase 24 results in theformation of a first nucleated phase of NiSi.

The stoichiometry of the first nucleated phase is dictated by theelemental composition adjacent the nucleation site(s). If a multilayerstructure 14, comprising roughly equivalent molar concentrations ofsilicon and nickel (as illustrated in FIG. 1), interdiffuses with anickel capping layer 16, the resulting amorphous phase 24 is relativelynickel-rich. A nickel-rich amorphous phase 24 also can be created byannealing a multilayer structure 14 in which the molar proportion of Niis greater than of Si, as illustrated in FIG. 2 in which the Ni layers20 are thicker (relative to the Si layers 22) than in the embodiment ofFIG. 1. Thus, the stoichiometry immediately surrounding the interfacebetween the nickel-rich amorphous phase 24 and the substantially puresilicon of the substrate 12 will dictate preferential formation of NiSiat the interface so long as crystallization begins at the interface.

Alternatively, if a multilayer structure 14, comprising roughlyequivalent concentrations of silicon and nickel, interdiffuses withitself but not with the nickel capping layer 16, NiSi formation stillwill be achieved so long as nucleation occurs at “embryonic” siteswithin the amorphous phase 24 rather than at the interface between thesubstrate 12 and the amorphous phase 24.

Using multilayer structures 14 as reactive precursors provides severaladvantages for preparing NiSi materials in microelectronic devices.First, the subject methods that result in formation of such regions canbe performed using equipment that is similar to equipment already used(for other purposes) in microelectronic-device-fabrication facilities.Second, the subject methods suppress formation of unwanted phasesbecause the desired phase forms directly from the amorphousintermediate.

Nucleation of a particular desired phase can be achieved by adjustingthe composition of the multilayer structure 14. The composition of themultilayer structure 14 can be controlled simply by controlling therespective deposition times for each elemental layer 20, 22. Thelocations of nucleation sites in the amorphous phase 24 and thestoichiometry of the amorphous phase 24 created by interdiffusion of thelayers 20, 22 overlying the substrate 12 are key factors in determiningthe stoichiometry of the first nucleated phase. To cause the formationof a desired crystalline phase, the composition of the layers 20, 22 isadjusted to cause the formation of the desired amorphous phase eitherimmediately upon deposition of the layers 20, 22 or during the firstannealing step. Depending on whether nucleation is expected to occur atthe interface between the substrate 12 and the amorphous phase 24 orwhether nucleation is expected to occur at embryonic sites within theamorphous phase 24, the desired composition of the amorphous phaseeither will be nickel-rich or will comprise approximately equalconcentrations of Ni and Si.

To illustrate, in the Ni—Si system illustrated in FIG. 1, the Ni:Sistoichiometry of the multilayer structure 14 is approximately 1:1. Ifannealing is performed to cause crystalline-phase nucleation beforesignificant diffusion of Ni from the capping layer 16 into themultilayer structure 14, the nucleations will occur at embryonic siteswithin the amorphous phase formed from the multilayer structure 14, andthe first crystalline phase will be NiSi. Whether nucleation will occurwithin the amorphous phase depends upon the thickness of the layer pairs16. During annealing, an amorphous phase begins to form at each Ni—Siinterface. If the thickness of any of the layer pairs 16 is excessive,nucleation will occur at the interface between the growing amorphousphase and the adjacent thicker layer pair 16. For example, if thematerial surrounding the nucleation sites in this scenario is mostly Ni(e.g., because nucleation occurs at interfaces between amorphous phasesand Ni layers), then the prevailing stoichiometry dictates formation ofan undesirably nickel-rich silicide. If, however, the thickness of eachlayer pair 16 is below a particular threshold, then each growing unit ofamorphous phase formed during annealing will come into contact withadjacently growing units of amorphous phase before onset ofcrystallization. Thus, a unified amorphous phase is formed that extendsbeyond the thickness at which crystallization otherwise would occur.Such an amorphous phase has no interface at which nucleation can occur.Hence, nucleation will occur only at embryonic sites within theamorphous phase in which the stoichiometry of the amorphous phasedictates the stoichiometry of the crystalline phase formed at thenucleation sites. For example, a layer-pair thickness of 2.5 nm or lesseffectively favors nucleation within a Ni—Si amorphous phase.

Alternatively, the annealing steps can be controlled to allow diffusionof the multilayer structure 14 with the nickel capping layer 16 beforeonset of crystallization. The first annealing step forms a nickel-richamorphous phase 24. A subsequent ramp-up in temperature initiatesnucleations at the interface between the amorphous phase 24 and thesilicon of the substrate 12. The stoichiometry surrounding thenucleation sites favors the formation of NiSi.

An advantage of using a multilayered structure of reactant layers isthat the constituent layers allow the formation of a desired crystallinephase at a lower temperature than conventionally. In this regard, thereare fundamental differences between the present multilayer-syntheticmethods and conventional solid-state reactions. In conventionalsolid-state reactions, complete mixing of the reactants is achievableonly at high temperatures. Crystalline-phase formation at the interfacebetween two reactants occurs prior to mixing of the reactants, whichfavors the formation of multiple crystalline “intermediate” phases enroute to the final product. These intermediate phases typically have norelationship, crystallographic or otherwise, to the desired finalproduct. In contrast, using methods as described above, the respectivelayers and layer pairs in the multilayer structure are sufficiently thinsuch that complete intermixture of reactant layers is achieved atmoderate temperatures, which produces a desired substantiallyhomogeneous amorphous phase prior to onset of nucleation of anycrystalline phases. The amorphous phase serves as a reactiveintermediate that is a direct precursor to formation of a correspondingcrystalline phase. By varying the composition of the multilayerstructure (i.e., by appropriate control of layer thicknesses), thecomposition of the amorphous precursor is controlled, yieldingcorresponding control of the particular final crystalline product thatis formed from the amorphous precursor.

The methods described above are especially useful in the manufacture ofmicroelectronic devices such as integrated circuits, displays, and thelike. A particularly advantageous application of the subject methods isin the fabrication of conductive contacts for use in making electricalconnections between, for example, active circuit elements and metalconductors in integrated circuits. In such fabrication methods, the locion active circuit elements at which such electrical connections are tobe made typically are defined microlithographically. Since the methodsdisclosed herein are performed at relatively low temperatures, they canbe conducted on patterned semiconductor wafers in many instances withoutdestroying masks and the like that define the loci.

The following example is provided to illustrate certain operationaldetails of a representative embodiment. It will be understood that thisexample is not limiting in any way and that other embodiments are notlimited to the particular details described in this example.

Specifically, in this example a composition comprising substantiallypure NiSi was formed on a silicon substrate. The substrate was formedfrom 100-mm diameter n-type Si(100) wafers (resistivity>6 Ω/cm). Thewafers were broken into pieces 1 cm×1 cm. The native oxide was strippedoff each piece using a conventional cleaning procedure. Within one hourof stripping the oxide, the silicon pieces were mounted onto scrap200-mm Si wafers using double-sided Scotch® tape and loaded into anevaporative physical vapor deposition (PVD) chamber (Thermionics, Inc.).Background pressure in the chamber was ˜10⁻⁷ Torr. Ni and Si weredeposited using respective electron guns and respective sources at anominal deposition rate of 0.15 nm/s, as determined using aquartz-crystal monitor. Real deposition rates of 0.04 nm/s for both Niand Si were confirmed by ex situ x-ray reflectivity.

Three samples were prepared. In each sample, several repeating units(layer pairs) of equimolar Ni and Si layers were deposited at athickness (per layer pair) of approximately 20 nm. This was followed bydepositing a capping layer of nickel at a thickness of approximately 30nm. Specific deposition details are listed in Table 1, below: TABLE 1 Nidep (s) Si dep (s) Target Target Thickness Thickness Ni Cap (s) (nm)(nm) Target Thickness (nm) Sample Thickness determined Repeat Thicknessdetermined ID by X-Ray Reflectivity Units by X-Ray Reflectivity J05218.8 s 37.7 s 2 667 s X_(Ni)˜0.95 0.72 nm 1.45 nm 25.7 nm 1.69 nm 28.7nm J053 18.8 s 37.7 s 5 520.8 s X_(Ni)˜0.86 0.72 nm 1.45 nm 20.1 nm 1.90nm 21.0 nm J054 18.8 s 37.7 s 10 260.4 s X_(Ni)˜0.70 0.72 nm 1.45 nm10.0 nm 2.07 nm 7.67 nmIn the foregoing Table multilayer depositions are in units of seconds(s) of deposition time for each constituent element. The term “repeatunits” is synonymous with “layer pairs.” The “nickel cap” is the cappinglayer of nickel, the formation of which is expressed in terms of secondsof deposition time. The variable “X_(Ni)” is the approximate molefraction of nickel in the entire overlayer (i.e., in the multilayerstructure plus in the nickel cap).

Grazing-angle diffraction data of the as-deposited samples are shown inFIG. 5, which reveals that polycrystalline Ni was present in all threesamples. The intensity of the polycrystalline Ni peaks correspondsqualitatively to the thickness of the Ni capping layer. Higher-intensitypeaks correspond to thicker caps. Other than peaks corresponding toscattering from the substrate and peaks arising from the capping layer,no other peaks were observed, indicating that the multilayer structurewas amorphous upon deposition.

The samples were annealed in a nitrogen atmosphere for one hour atsuccessively higher temperatures. After each annealing step, x-rayreflectivity and grazing-angle diffraction were used to monitor thecourse of the reaction. Annealing was continued until crystalline-phasenucleation was observed in the grazing-angle-diffraction data. Annealingresults are shown in FIG. 6 for sample J052, in FIG. 7 for sample J053,and in FIG. 8 for sample J054.

In sample J052, as deposited, a significant amount of crystalline Ni wasobserved (FIG. 6). The intensity of the Ni peaks on the reflectivitycurve decreased upon annealing. This resulted from the amorphoussilicide consuming the available Ni. At 300° C. the diffraction patternshowed a mixture of orthorhombic Ni₂Si (marked “•” in FIG. 6) and theremaining crystalline Ni (marked “□” in FIG. 6). The peak marked “*” inFIG. 6 represents the substrate. The phase evolution observed in sampleJ052 was similar to that observed in a traditional Ni—Si diffusioncouple.

In sample J053, polycrystalline Ni again was the predominant phase asdeposited (FIG. 7). At 250° C. nucleation was observed. At 300° C., amixture of multiple phases was observed, including the remains of theinitial Ni layer. Among the phases formed was orthorhombic NiSi.

In sample J054, the Ni was thinner (˜10 nm); thus, the polycrystallineNi phase was not as pronounced as in the as-deposited sample (FIG. 8).Below 200° C. the diffraction was largely unchanged, with the onlyapparent peaks belonging to polycrystalline Ni. Upon annealing to 200°C. the Ni peaks began to broaden, while higher-order peaks disappearedaltogether. By 250° C. the peak had broadened significantly, suggestingsignificant interdiffusion of the Ni and NiSi layers. At 300° C. thepeaks increased in intensity and shifted to higher angles, wherein theincrease in intensity suggests further homogenization of the amorphousphase. Further annealing to 350° C. resulted in the nucleation of NiSi,with no other detectable phases being present.

Whereas the invention has been described above in connection withmultiple representative embodiments, the invention is not limited tothose embodiments. On the contrary, the invention encompasses allmodifications, alternatives, and equivalents as may be included withinthe spirit and scope of the invention, as defined by the appendedclaims.

1. A composition of matter, comprising: a substrate having a surface;and a multilayer structure formed on the surface of the substrate, themultilayer structure comprising multiple superposed layer pairs, eachlayer pair consisting of a first layer of silicon and a second layer ofnickel and having a layer-pair thickness of 3.0 nm or less.
 2. Thecomposition of claim 1, wherein the substrate is selected from the groupconsisting of semiconductor materials, metals, glass materials,crystalline materials, and ceramic materials.
 3. The composition ofclaim 2, wherein the surface of the substrate is a surface of a siliconlayer applied to the substrate.
 4. The composition of claim 1, whereinthe substrate is silicon.
 5. The composition of claim 1, exhibiting anelectrical conductivity, from the multilayer structure to the substrate,of less than 13 μΩ·cm.
 6. The composition of claim 1, wherein themultilayer structure comprises two to ten layer pairs.
 7. Thecomposition of claim 1, wherein the multilayer structure comprises morethan 50 mole-percent of nickel.
 8. The composition of claim 1, whereinthe multilayer structure comprises substantially equal mole percentagesof silicon and nickel.
 9. The composition of claim 1, further comprisinga capping layer superposed on the multilayer structure.
 10. Thecomposition of claim 7 wherein the capping layer is a layer of nickel.11. The composition of claim 1, wherein the multilayer structure is anamorphous phase of silicon and nickel.
 12. The composition of claim 11wherein the multilayer structure is amorphous NiSi.
 13. The compositionof claim 12 further comprising a metal capping layer superposed on themultilayer structure.
 14. The composition of claim 13 wherein the metalis nickel.
 15. The composition of claim 1, wherein the multilayerstructure is crystalline SiNi.
 16. The composition of claim 15, furthercomprising a nickel capping layer.
 17. A method for making a compound ofnickel and silicon, comprising: on a surface of a substrate, formingmultiple layer pairs in a superposed manner, each layer pair comprisinga respective layer of nickel and a respective layer of silicon eachbeing 3 nm or less in thickness, wherein the layers of nickel andsilicon in the multiple layer pairs are formed in alternating order,thereby forming a multilayer structure, wherein the layers of nickel andsilicon in the multilayer structure are formed at respective thicknessescorresponding to desired mole fractions of nickel and silicon in themultilayer structure; annealing the multilayer structure at an annealingtemperature of 200° C. or less to form an amorphous alloy of nickel andsilicon in the multilayer structure, the alloy having the desired molefractions of nickel and silicon; and allowing the amorphous alloy tonucleate and form a corresponding crystalline alloy.
 18. The method ofclaim 17, wherein the crystalline alloy has the desired mole fractionsof nickel and silicon
 19. The method of claim 17, wherein the step ofallowing the amorphous alloy to nucleate is performed by annealing theamorphous alloy at an annealing temperature of 350° C. or less.
 20. Themethod of claim 19, wherein the step of annealing the amorphous alloycomprises, at onset of annealing, ramping up to the annealingtemperature of 350° C. or less.
 21. The method of claim 17, wherein thestep of forming the multiple layer pairs is performed on a substrateselected from the group consisting of semiconductor materials, glassmaterials, ceramic materials, crystalline materials, and metalmaterials.
 22. The method of claim 17, wherein the step of forming themultiple layer pairs is performed on a substrate having a siliconsurface.
 23. The method of claim 22, further comprising the step ofcleaning the silicon surface before forming the multilayer structure onthe silicon surface.
 24. The method of claim 22, wherein the substrateis silicon.
 25. The method of claim 17, wherein the step of forming themultiple layer pairs comprises forming two to ten layer pairs.
 26. Themethod of claim 17, wherein the layers of silicon and nickel are formedat respective thicknesses sufficient to form the multilayer structurehaving substantial equal mole percentages of nickel and silicon.
 27. Themethod of claim 17, wherein the layers of silicon and nickel are formedat respective thicknesses sufficient to form the multilayer structurehaving more than 50 mole-percent of nickel.
 28. The method of claim 17,further comprising the step of forming a capping layer superposedly onthe multilayer structure.
 29. The method of claim 28, wherein thecapping layer is a layer of nickel.
 30. The method of claim 17, whereineach of the nickel layers and each of the silicon layers is formed byelectron beam evaporation.
 31. A compound of nickel and silicon formedby the method recited in claim
 17. 32. The compound of claim 31, whereinthe compound is a crystalline alloy of silicon and nickel.
 33. In amicroelectronic-device fabrication method, a method for providing asilicon-containing active-circuit element with a low-resistivitycontact, the method comprising: on a region of the surface of theactive-circuit element, forming multiple layer pairs in a superposedmanner, each layer pair comprising a respective layer of nickel and arespective layer of silicon each being 3 nm or less in thickness,wherein the layers of nickel and silicon in the multiple layer pairs areformed in alternating order, thereby forming a multilayer structure,wherein the layers of nickel and silicon in the multilayer structure areformed at respective thicknesses corresponding to desired mole fractionsof nickel and silicon in the multilayer structure; annealing themultilayer structure at an annealing temperature of 200° C. or less toform an amorphous alloy of nickel and silicon in the multilayerstructure, the alloy having the desired mole fractions of nickel andsilicon; and allowing the amorphous alloy to nucleate and form acorresponding crystalline alloy.
 34. The method of claim 33, wherein thecrystalline alloy has the desired mole fractions of nickel and silicon.35. The method of claim 33, further comprising the step, after formingthe crystalline alloy, of connecting a metal conductor to thecrystalline alloy so as to establish a low-resistivity contact betweenthe active-circuit element and the metal conductor.
 36. The method ofclaim 33, further comprising the step of forming a capping layer on themultilayer structure before annealing the multilayer structure.
 37. Themethod of claim 36, wherein the capping layer is a layer of nickel. 38.The method of claim 36, further comprising the step, after forming thecrystalline alloy, of connecting a metal conductor to the crystallinealloy so as to establish a low-resistivity contact between theactive-circuit element and the metal conductor.
 39. The method of claim33, wherein the step of allowing the amorphous alloy to nucleate isperformed by annealing the amorphous alloy at an annealing temperatureof 350° C. or less.
 40. The method of claim 39, wherein the step ofannealing the amorphous alloy comprises, at onset of annealing, rampingup to the annealing temperature of 350° C. or less.
 41. The method ofclaim 33, further comprising the step of cleaning the surface of theactive-circuit element before forming the multilayer structure on thesilicon surface.
 42. The method of claim 33, wherein the step of formingthe multiple layer pairs comprises forming two to ten layer pairs. 43.The method of claim 33, wherein the layers of silicon and nickel areformed at respective thicknesses sufficient to form the multilayerstructure having substantial equal mole percentages of nickel andsilicon.
 44. The method of claim 33, wherein the layers of silicon andnickel are formed at respective thicknesses sufficient to form themultilayer structure having more than 50 mole-percent of nickel.
 45. Themethod of claim 33, wherein each of the nickel layers and each of thesilicon layers is formed by electron beam evaporation.
 46. Amicroelectronic device, comprising low-resistivity contacts formed asrecited in claim 33.